Does Blender have a method to a get PNG-formatted bytearray for an image via Python?
I got this working based on Python code from Stack Overflow here.

Here's the version of that code adapted to work inside stock Blender 2.77a:

import base64
import zlib
import struct

def img_to_png(blender_image):
    width = blender_image.size[0]
    height = blender_image.size[1]
    buf = bytearray([int(p * 255) for p in blender_image.pixels])

    # reverse the vertical line order and add null bytes at the start
    width_byte_4 = width * 4
    raw_data = b''.join(b'\x00' + buf[span:span + width_byte_4]
                        for span in range((height - 1) * width_byte_4, -1, - width_byte_4))

    def png_pack(png_tag, data):
        chunk_head = png_tag + data
        return (struct.pack("!I", len(data)) +
                chunk_head +
                struct.pack("!I", 0xFFFFFFFF &amp; zlib.crc32(chunk_head)))

    png_bytes = b''.join([
        b'\x89PNG\r\n\x1a\n',
        png_pack(b'IHDR', struct.pack("!2I5B", width, height, 8, 6, 0, 0, 0)),
        png_pack(b'IDAT', zlib.compress(raw_data, 9)),
        png_pack(b'IEND', b'')])

    return 'data:image/png;base64,' + base64.b64encode(png_bytes).decode()


Example usage:

print(img_to_png(bpy.data.images[0]))


This will print out a PNG-format data-URI of the image, regardless of what format the image is within Blender, and without altering the user's image.  I suggest using a very small image to test with, so the test output is not overwhelming when printed to the console.